1. Field of the Invention
The invention relates to a method and an apparatus for the rapid melting of in particular high-purity, aggressive and high-melting glasses in a skull crucible, in which high-frequency energy is introduced into the contents of the crucible by means of a coil arrangement surrounding the skull crucible, in order to heat the melt, and the batch is laid and the molten glass discharged in the upper region of the crucible, and undissolved constituents of the batch are retained by means of a cooled bridge which is at least partially immersed in the melt from above.
2. Description of Related Art
Nowadays, aggressive glasses which are required to have a high purity are melted discontinuously in platinum crucibles or continuously in platinum tank furnaces. Both the melting tank furnace and the refining tank furnace and the homogenization tank furnace consist of platinum. This melting technology is disadvantageous on account of the high costs of the precious metal and also the short service lives of platinum equipment of this type. In particular the melting region, where batch reactions take place, is exposed to considerable corrosion and is therefore often the limiting component of a platinum tank furnace in terms of the service lives. On account of the heating technology via the platinum wall and the stability of the platinum, the maximum throughputs which can be achieved with equipment of this type are less than 1 t per day for a melting tank furnace or crucible volume of 90 l.
In addition to platinum melting units, it is also known to use skull crucibles, which are formed from water-cooled, spaced-apart metal tubes and in which the melt is heated by means of induction coils which surround the crucible, by high-frequency energy being radiated in. Melting equipment of this type has the advantage that the water cooling means that a protective layer of material of the same composition of the glass is inevitably formed in the edge region of the crucible, so as to surround the melt in the form of a crucible of the same composition of the glass, so as to provide protection against impurities.
PETROV, YU. B. ET AL.: “Continuous casting glass melting in a cold crucible induction furnace”, XV INTERNATIONAL CONGRESS ON GLASS 1989, PROCEEDINGS, Vol. 3a, 1989, pages 72-77 has disclosed a crucible of the abovementioned type for melting high-purity glasses. In this crucible, the batch is supplied in the upper crucible region and the glass is likewise taken off in the upper crucible region. The batch and outlet region are separated from one another by a cooled bridge, which is immersed deep in the melt, in order to retain undissolved constituents of the batch. The melted glass is taken off at the upper edge of the crucible via an overflow channel arranged inside the coil and drops downward in the form of a glass strand between the crucible wall and the internal radius of the coil.
The document does not give any information as to how the glass strand is collected and then fed for further processing. However, it is obvious that in the arrangement described the possibilities for connecting the melting unit to the further-processing units are very limited. Moreover, with the known procedure it is likely that the quantity of glass in the glass strand will be subject to fluctuations over the course of time, so that at best only a quasi-continuous procedure is possible. A further drawback is that the drop height of the glass strand has to be very great, since the glass has to drop through at least the entire height of the coil before it can be collected in a channel or tank furnace arranged outside the coil. Consequently, it is likely that bubbles will be introduced into the melt and that the quality with regard to cords will deteriorate. Furthermore, cooling of the glass in the glass strand may be problematic in the case of high-melting glasses. The problem may be that the glass is not guided and therefore starts to splash. Furthermore, sparkovers may occur between coil and glass strand or between glass strand and crucible, which can lead to destruction of the coil and/or of the crucible.
Furthermore, document FR-A 2 561 761 has disclosed an apparatus with an inductively heated, cold melting crucible for the ongoing removal of melted substances. The melt is removed via a discharge made from refractory material. Moreover, an inclined, pivotable retaining device is provided in the melting crucible, in front of the discharge.
Document FR-A 2 589 228 shows a similar apparatus for the continuous production of materials which are obtained from substances in the molten state. In this apparatus, the material overflows continuously from a cold melting crucible, via a pipeline or channel, into a vessel.
However, the abovementioned apparatuses are in need of further improvement, in particular with regard to the melting capacity and the glass quality which can be achieved.